This application claims benefit of priority under 35 U.S.C. xc2xa7119 to Japanese Patent Applications No.2000-139692, filed on May 12, 2000, and No.2001-75691, filed on Mar. 16, 2001, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus for reproducing digital information from an optical disc medium in which the digital information is modulated and recorded in a recording track formed coaxially or spirally according to signal modulation method for restricting run length for limiting the recording mark length, and more particularly to an optical disc reproducing apparatus having a laser of 390 to 440 nm in wavelength for reproducing information from a high density read only memory (ROM) or from rewritable disc in which the information is recorded such that a recording mark in the optical disc medium is 0.32 xcexcm or more to less than 0.38 xcexcm in track pitch and 0.12 xcexcm or more to less than 0.22 xcexcm in mark length.
2. Description of the Related Art
Since before, as a means for reproducing information from a high-density ROM disc, a reproducing apparatus provided with a combination of a partial response equalization and viterbi decoding using run length restriction in recording modulation has been disclosed in, for example, Pioneer RandD Vol. 6. No.2 xe2x80x9cDevelopment of Viterbi Decoder for DVDxe2x80x9d and Japanese Patent Application Laid-Open No. H10-106161.
Although prior arts disclosed in these document and publication contribute largely to a reproducing apparatus for reproducing from a medium having a recording density of DVD or similar capacity, in a reproducing apparatus for reproducing from a higher density ROM (read only) disc (hereinafter sometimes referred to as simply ROM disc), nonlinear distortion caused in pit formation and cross-talk between adjacent tracks are increased tremendously, so that no sufficient margin can be secured due to disc tilt and the like, thereby making it impossible to construct any reproducing apparatus. Further, the rewritable disc has a similar problem caused by a recording power deviation or the like.
To solve the above described problem, the present invention intends to provide an optical disc reproducing apparatus capable of suppressing nonlinear distortion caused by pit formation, an influence by cross-talk between adjacent tracks, an influence by disc tilt and the like to a minimum extent in a high-recording density ROM disc in which a recording mark is 0.32 xcexcm or more to less than 0.38 xcexcm in track pitch and 0.12 xcexcm or more to less than 0.22 xcexcm in mark length, the apparatus using laser beam of 390 to 440 nm in wavelength.
0.22 xcexcm and 0.38 xcexcm which are upper limits of the length dimension and track pitch respectively are specified as values allowing recording of information with practically sufficient capacity for an optical disc medium of 12 cm in diameter. On the other hand, 0.12 xcexcm is a lower limit of the length dimension and is specified as a value within its practical limit not to increase jitters rapidly due to thermal interference between adjacent marks. Further, 0.32 xcexcm, which is a lower limit of the track pitch is a value specified by manufacturing limit for the disc.
To achieve the above object, there is provided an optical disc reproducing apparatus for reproducing digital information from an optical disc medium in which the information is modulated and recorded in a recording track formed coaxially or spirally according to a signal modulation method for restricting run length such that a recording mark is 0.32 xcexcm or more to less than 0.38 xcexcm in track pitch and 0.12 xcexcm or more to less than 0.22 xcexcm in mark length, comprising: laser beam source for emitting laser beam having wavelength of 390 to 440 nm; a reflection light detecting means for detecting a reflection light produced when the laser beam emitted from the laser beam source is reflected by the optical disc medium to read a signal from the optical disc medium, so as to reproduce the information recorded in the optical disc medium; a digital phase synchronizing means for receiving a supply of signal outputted from the reflection light detecting means and synchronizing digital phase so as to detect a phase at a data point and estimate digital data at the data point and outputting the digital data; an adaptive equalizing means including a transversal filter for equalizing in terms of waveform, the digital data outputted from the digital phase synchronizing means based on a filter coefficient, a temporary determining/subtracting circuit for while a sample after the waveform equalization outputted from the transversal filter is inputted therein, computing a temporary determining value for partial response equalization by comparing the sample with at least one threshold and outputting a differential between the temporary determining value and the sample after the waveform equalization as an error signal, and a coefficient control circuit for variably controlling a tap coefficient of the transversal filter so that the error signal becomes minimum; and a viterbi decoding means for viterbi-decoding a sample outputted from the adaptive equalizing means.
According to a preferred embodiment of the present invention, the coefficient control circuit includes an error selecting circuit for while the error signal and the temporary determining value outputted from the temporary determining means are inputted therein, selecting and outputting only valid component of the error signal depending on the temporary determining value, and a multiplier/low-pass-filter for variably controlling the tap coefficient of the transversal filter based on a signal outputted from the error selecting means such that the error signal becomes minimum.
According to another preferred embodiment of the present invention, an estimation value generating means for generating an estimation value by averaging output sample of the transversal filter for each of the at least one threshold based on the temporary determining value and the error signal outputted from the temporary determining/subtracting circuit is further comprised.
According to still another preferred embodiment of the present invention, the viterbi decoding means includes an input means for an estimation value for use in metric operation, the estimation value being capable of being controlled through the input means.
According to yet still another preferred embodiment of the present invention, the estimation value for the viterbi decoding means is provided by the estimation value generating means.
To achieve the above object, there is provided an optical disc reproducing apparatus for reproducing digital information from an optical disc medium in which the information is modulated and recorded in a recording track formed coaxially or spirally according to a signal modulation method for restricting run length such that a recording mark is 0.32 xcexcm or more to less than 0.38 xcexcm in track pitch and 0.12 xcexcm or more to less than 0.22 xcexcm in mark length, comprising: laser beam source for emitting laser beam having wavelength of 390 to 440 nm; a first reflection light detecting means for detecting a reflection light produced when the laser beam emitted from the laser beam source is reflected by the optical disc medium to read a signal from the optical disc medium, so as to reproduce the information recorded in the optical disc medium; a second reflection light detecting means for reading information separately from each of at least one pair of recording tracks on both sides of the recording track; a digital phase synchronizing means for while a signal outputted from the first reflection light detecting means is supplied thereto, digitally synchronizing phase so as to detect a phase at a data point and estimate digital data at the data point and outputting the digital data; a resampling means for while a signal outputted from the second reflection light detecting means is supplied thereto, resampling a signal from at least one recording track of the at least one pair of the recording tracks at the data point in the digital phase synchronizing means; an adaptive equalizing means including a first transversal filter for equalizing in terms of waveform, the digital data outputted from the digital phase synchronizing means based on a first filter coefficient, a temporary determining/subtracting circuit for while a sample after the waveform equalization outputted from the first transversal filter is inputted therein, computing a temporary determining value for partial response equalization by comparing the sample with at least one threshold and outputting a differential between the temporary determining value and the sample after the waveform equalization as an error signal, a second transversal filter for equalizing in waveform, the digital data outputted from the resampling means based on a second filter coefficient and generating a pseudo cross-talk signal based on an inputted tap coefficient, a subtracting means for subtracting the pseudo cross-talk signal from the sample after the waveform equalization, and a coefficient control circuit for variably controlling the tap coefficient inputted to the first and second transversal filters so that the error signal becomes minimum; and a viterbi decoding means for viterbi-decoding a sample outputted from the adaptive equalizing means.
According to a preferred embodiment of the present invention, the first coefficient control circuit includes an error selecting circuit for while the error signal and the temporary determining value outputted from the temporary determining means are inputted therein, selecting and outputting only valid component of the error signal depending on the temporary determining value, and a multiplier/low-pass-filter for variably controlling the tap coefficient of the first and second transversal filters based on a signal outputted from the error selecting means such that the error signal becomes minimum.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.